Field of the Invention
The invention relates to a sensor for determining the course of concentration of an adsorbent substance, such as ammonia, that diffuses out of a flow medium through a surface of a sensor material into the sensor material, for determining the course as a function of the distance from the surface.
In the catalytic conversion of at least two reagents of a flow medium, if high degrees of settling are to be attained, it is necessary to put the reagents into contact with a catalyst in a suitable stoichiometric ratio to one another. In the case of the catalytic conversion of nitrogen oxides contained in an exhaust gas or flue gas by the method of selective catalytic reduction (SCR) with ammonia as the reducing agent, for instance, it is necessary for the nitrogen oxides and ammonia to be present at the catalyst in approximately equal portions, averaged over time. Adjusting the requisite stoichiometric ratio of nitrogen oxides to ammonia, or to some substance such as urea that can be converted into ammonia, can be done with satisfaction only if the nitrogen oxide concentration in the exhaust or flue gas can be measured, or can be determined comparatively accurately through a performance graph diagnosis.
However, measuring the nitrogen oxide concentration requires comparatively major effort and entails comparatively great expense. For instance, such measurements can be carried out only in large SCR systems in power plants, but once again a local leakage of ammonia caused by nitrogen oxide skewing can be only unsatisfactorily prevented. Yet avoiding leakage of ammonia must be sought under all circumstances, since ammonia is poisonous and in even extremely slight concentrations ammonia causes annoyance due to human beings to its smell (odor threshold approximately 5 ppm).
Since the nitrogen oxide concentration in fossil-fueled power plant flue gases also only varies slowly when averaged over time, because of load changes that are only slowly completed over time, ammonia metering is carried out with the aid of complicated measurements of nitrogen oxide and optionally of ammonia as well, which measurements have relatively long time constants. Only ammonia metering for flue gases with slowly completed changes in nitrogen oxide concentration can thus be considered to have been achieved somewhat satisfactorily. Removing nitrogen from flue gases emitted by Diesel engines and lean-running engines is significantly more difficult. Due to different operating states and rapid load changes in those engines, a system constructed for those engines with a "controlled Diesel catalyst" (CDC), must be constructed for the resultant major fluctuations in the volumetric flow of exhaust gas, exhaust gas temperature, and nitrogen oxide concentration in the exhaust gas. Since ammonia itself is hazardous and therefore cannot be carried in vehicles, such as passenger cars, trucks, buses, locomotives and ships, the requisite reducing agent is instead carried in the vehicle in the form of an aqueous urea solution, for instance, from which ammonia is then generated, ideally in precisely the quantity needed at the moment.
Since the use of currently known nitrogen oxide sensors in vehicles is extremely improbable because of the major effort and expense involved, the attempt has been made instead to ascertain the quantities of nitrogen oxide produced by the engine per unit of time by a performance graph comparison, adding metered quantities of a suitable reducing agent to those thus calculated quantities of nitrogen oxide, and simulating the settling capacity of the catalyst in an on-board diagnosis system (see German Published, Non-Prosecuted Application DE 43 15 278 A1, for instance). Thus, for the metered addition of the reducing agent in the CDC system, there is thus far no suitable control capability, and as a result if there are defects in the catalyst or at the metering valve, annoyance can be created by excessive leakage of ammonia.